Well over 100,000 megawatt and multi-megawatt wind turbines have been installed over the past decade, almost all using a similar drive system incorporating a gearbox as a speed increaser, positioned between the turbine blades and the generator. The gearboxes are designed for 20-year life, but typically need repair or replacement in 5 to 10 years or less. Axial cracking of gearbox bearings is becoming a major cost factor in the return on investment of wind farms. Impact loading during transient torque reversals has been recognized as a root cause of this damage. Recently research has shown that an unusual mode of bearing damage called White Etch Area (WEA) damage is causing the axial cracking of the bearings. WEA damage is actually a microscopic material alteration that creates super-hard inclusions like slivers just below the bearing raceway where cracks can initiate and grow. Severe and rapid microscopic plastic deformation is suspected as the cause of WEA damage.
During a torque reversal, the load zone of the gearbox bearings suddenly shifts 180 degrees. The bearing rollers radially impact onto the raceway along with a simultaneous high axial load reversal from the helical gears. Both the magnitude and the rate of the impact loads and axial surface traction loads determine the potential for WEA plastic deformation in the bearing inner race. The higher the torsional natural frequency of the drive train's spring mass system, the greater the torque rate of change, and thus the higher the strain rate as the rollers impact the bearing inner race . As wind turbines have increased in size, the high strain rate during rapid bearing load zone reversals, along with high impact stress, appears to be exceeding a threshold where WEA damage is initiated in the bearing inner raceway. Once initiated the normal roller loading can cause axial cracking and bearing failures in as little as a year or two.
In a wind turbine generator system, high inertia characterizes the entire system, from the turbine blades, main shaft, gearbox high-speed coupling and into the generator itself. Indeed, the highest inertia is typically at the opposite ends of the system—at the blades and the generator. On torque reversal, the high inertia of the system can significantly impact all of the system components, and particularly the gearbox. The asymmetric torque-limiting clutch system described in U.S. Patent Application Publication US 2012/045335 A1 describes a solution to this problem. An alternate solution contemplates increasing the torsional wind-up of the system (including certain of, blades, a main shaft, a gearbox, a high-speed shaft/coupling and a generator) which would lower the natural frequency. If this were done alone, it could cause other problems in the turbine drive system; such as resonant frequency issues in other parts of the turbine. For instance, it is known that the coupling spacer between the gearbox and the generator can have a problematic axial natural frequency that can cause spacer element resonance and destruction. Any changes to the system natural frequency during normal operation may necessitate a recertification of the turbine.
Increasing the torsional wind-up must be done in a way that does not affect normal operation of the turbine. This could be accomplished with a high frictional slip ability in parallel with a high torsional wind-up and/or displacement ability. For example, if the frictional torque setting was at 40% of the rated turbine torque, there would be no slippage during normal operation between 20% and 100% of the rated turbine torque as apparent from FIG. 1. The only time the friction slippage would occur is when the drive system sees a total torque variation exceeding 80% of the rated turbine torque, for example for brief periods during startups and shutdowns. Significant slippage would only occur during rare system transient torque reversals exceeding the frictional slip setting. It is contemplated that the frictional torque setting should be such as to accommodate some small slippage during normal startup and shutdown operation to keep the friction surfaces clean and free of corrosion.
If the high torsional wind-up is effected by a torsional spring, as contemplated in an embodiment of the invention, the torsional spring rate may be asymmetric so that the spring rate in reverse could be lower or near zero for a portion of the displacement. Any reverse torque events would slip at frictional resistance only of say 40% of normal turbine torque. The reverse angle of travel would need to be sufficient to absorb reverse transient wind-up energy of the drive system. This may require a torsional movement of 10 to 50 degrees or greater for typical turbines with generators operating at 1000 rpm or more. For turbines with lower generator operating speeds, the required torsional displacement would be lower, in the range of 1 to 5 degrees per 100 rpm.
The typical coupling systems of existing wind turbines are designed with significant parallel, angular and axial shaft misalignment capability between the gearbox and the generator in order to accommodate the flexing of the lightweight base plate structure. These coupling systems typically have zero backlash and are torsionally very rigid with very little wind-up ability. The torsional characteristics are critically important to preventing resonant vibration problems in the drive system and turbine components. Some coupling systems are equipped with frictional torque limiters set at 150 to 200% of the rated turbine torque. They are intended to protect the coupling from the very high torque overloads such as generator short circuits. These torque limiters have proved to be ineffective in protecting the drive system and especially the gearbox from transient torque reversals whose impact loads on the gearbox bearings can dramatically shorten life.
Coupling systems that utilize torsional wind-up in parallel with low frictional damping, such as Spaetgens U.S. Pat. No. 2,909,911 and Lech U.S. Pat. No. 4,5548,311, have been around for a long time. They are generally used on internal combustion engines. Their torsional wind-up ability is used to tune the natural frequencies of the system to be outside the operating range of the equipment. Their frictional damping component that is in parallel with the torsional wind-up is typically very small and is used to control clutch plate and gear rattle noise and damage during idling and shifting. These types of couplings generally are integrated with the engine clutch whose frictional slip setting is very high and is in series with the torsional wind-up ability, not in parallel. Lech is a good example. The frictional component that is in parallel has a very low frictional slip setting.
A key to the present invention's success is a coupling system with a high torsional wind-up and/or displacement ability, along with a high frictional slip ability to dampen the system significantly only during a transient torque reversal event (see FIG. 1). A typical turbine with a high speed generator operating at 1000 to 1800 rpm would require at least 10 degrees of reverse slippage with a torque setting of at least 10% of turbine rated torque. Ideally, the reverse slippage would exceed 20 degrees at 40% reverse torque. Nowhere in the prior art is there a drive system with such a combination of torsional displacement and/or wind-up with torsional frictional damping capable of taming high torque reversals. This is certainly not true for the uniquely challenging reversals of wind turbines.